Method and device for increasing the data transmission capacity in a serial bus system

ABSTRACT

A method is described for serial data transmission in a bus system having at least two subscribed data processing units that exchange messages via the bus, the transmitted messages having a logical structure in accordance with CAN standard ISO 11898-1, the logical structure including a start-of-frame bit, an arbitration field, a control field, a data field, a CRC field, an acknowledge field and an end-of-frame sequence, the control field including a data length code, which contains information regarding the length of the data field. When a first switchover condition is satisfied, the data field of the messages, in contrast to CAN standard ISO 11898-1, may comprise more than eight bytes, the values of the four bits of the data length code being interpreted at least partially in deviation from CAN standard ISO 11898-1 for determining the size of the data field when the first switchover condition is satisfied.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage entry of InternationalPatent Application No. PCT/EP2012/055588, filed on Mar. 29, 2012, whichclaims priority to Application Nos. DE 10 2011 006 884.8, filed in theFederal Republic of Germany on Apr. 6, 2011, DE 10 2011 078 266.4, filedin the Federal Republic of Germany on Jun. 29, 2011, and DE 10 2011 080476.5, filed in the Federal Republic of Germany on Aug. 5, 2011.

FIELD OF INVENTION

The present invention relates to a method and a device for increasingthe data transmission capacity between at least two subscribers in aserial bus system.

BACKGROUND INFORMATION

For example, the ISO standard family 11898-1 through -5 describes theController Area Network (CAN) as well as an extension of the CAN called“time-triggered CAN” (TTCAN), referred to in the following also asstandard CAN. The media access control method used in the CAN standardis based on a bit-wise arbitration. In bit-wise arbitration, multiplesubscriber stations are simultaneously able to transmit data via thechannel of the bus system, without thereby interfering with the datatransmission. Furthermore, the subscriber stations are able to ascertainthe logical state (0 or 1) of the channel while transmitting a bit overthe channel. If a value of the transmitted bit does not correspond tothe ascertained logical state of the channel, the subscriber stationterminates the access to the channel. In CAN, the bit-wise arbitrationis usually carried out on the basis of an identifier within a messagethat is to be transmitted via the channel. After a subscriber stationhas sent the identifier to the channel in its entirety, it knows that ithas exclusive access to the channel. The end of the transmission of theidentifier thus corresponds to a beginning of an enable interval, withinwhich the subscriber station is able to use the channel exclusively.According to the CAN protocol specification, other subscriber stationsmay not access the channel, that is, send data to the channel, until thesending subscriber station has transmitted a checksum field (CRC field)of the message. Thus, an end point of the transmission of the CRC fieldcorresponds to an end of the enable interval.

The bit-wise arbitration thus achieves a non-destructive transmission ofthose messages via the channel that won the arbitration process. The CANprotocols are particularly suited for transmitting short messages underreal-time conditions, a suitable assignment of the identifiers beingable to ensure that particularly important messages will almost alwayswin the arbitration and be sent successfully.

With the increasing networking of modern vehicles and the introductionof additional systems for improving driving safety for example ordriving comfort, the demands grow on the quantities of data to betransmitted and the latency periods admissible in the transmission.Examples are driving dynamics control systems such as, e.g., theelectronic stability program ESP, driver assistance systems such as,e.g., the automatic distance control ACC, or driver information systemssuch as, e.g., the traffic sign detection (cf. for example descriptionsin “Bosch Kraftfahrtechnisches Handbuch” [“Bosch Automotive Handbook”],27th edition, 2011, Vieweg+Teubner).

German Application No. DE 103 11 395 describes a system in whichasynchronous, serial communication is able to take place alternativelyvia an asymmetrical physical protocol or via the symmetrical physicalCAN protocol, and thereby a higher data transmission rate or datatransmission reliability is achievable for the asynchronouscommunication.

German Application No. DE 10 2007 051 657 provides for the use of anasynchronous, fast, not CAN-compliant data transmission in the exclusivetime windows of the TTCAN protocol in order to increase the transmitteddata quantity.

G. Cena and A. Valenzano, in “Overclocking of controller area networks”(Electronics Letters, vol. 35, No. 22 (1999), p. 1924) deal with theeffects of overclocking the bus frequency in subsections of the messageson the effectively achieved data rate.

It is clear that the related art does not provide results that aresatisfactory in every respect.

SUMMARY

In the following, the present invention and its advantages will bedescribed with reference to drawings and exemplary embodiments. Thesubject matter of the present invention is not limited to therepresented and described exemplary embodiments.

The present invention is based on the transmission of messages having alogical structure according to the CAN standard ISO 11898-1 in a bussystem having at least two subscribed data processing units, the logicalstructure including a start-of-frame-bit, an arbitration field, acontrol field, a data field, a CRC field, an acknowledge field and anend-of-frame sequence, and the control field including a data lengthfield, which contains an item of information regarding the length of thedata field.

By providing a possibility for enlarging the data field of a transmittedmessage, the present invention achieves the effect that a greaterquantity of data may be transmitted across the bus by a single messagecompared to a standard-conforming CAN in that, when a first switchovercondition is satisfied, the data field of the messages may comprise morethan eight bytes in deviation from the CAN standard ISO 11898-1, thevalues of the bits of the data length code being interpreted at leastpartially in deviation from the CAN standard ISO 11898-1 for determiningthe size of the data field when the first switchover condition issatisfied. This advantageously increases the ratio of data quantity andcontrol information in a message and thus also the average datatransmission rate over the bus system.

By establishing an unequivocal correlation between the content of thedata length code and the length of the data field, a high flexibility isadvantageously achieved with respect to the possible size of the datafield.

It is furthermore advantageous that for the values 0b0001 through 0b1000of the data length code normally used in standard CAN, the sizes of thedata field corresponding to the CAN standard, that is, 1 byte through 8bytes, are assigned and the remaining values of the data length code areused for the additional admissible sizes of the data field up to themaximum admissible size. This cost-effectively reduces the adaptationeffort of application software when switching to the method according tothe present invention.

The enlargement of the data field and the adaptation of theinterpretation of the content of the data length code occurs as afunction of a first switchover condition such that when the switchovercondition is satisfied, the method according to the present invention isapplied, while otherwise the data transmission occurs according to thenormal CAN standard. This makes it possible to use devices according tothe present invention both in standard CAN bus systems as well as in newbus systems according to the present invention having potentiallygreater data fields.

Additionally, there may be a provision such that, when a secondswitchover condition is satisfied, the bits of the data length code areinterpreted at least partially in deviation from the CAN standard ISO11898-1 and also in deviation from the assignment that is made when theadditional switchover condition is not satisfied. Messages transmittedin this manner may be made recognizable by a second identification inthe arbitration field and/or in the control field. This achieves an evengreater flexibility with respect to the selection of valid sizes of thedata field. The second identification is evaluated in the subscribeddata processing units to ascertain the second switchover condition, andthe receiving process is adapted to the size of the data field as afunction of the second switchover condition.

It may be advantageous with regard to the modification effort comparedto the standard CAN protocol that the second identification appears onlyin messages, whose arbitration field has the format of a CAN message inextended format, and/or agrees with the identification of the extendedformat.

In the event that the data field is enlarged in accordance with thepresent invention, it is furthermore possible to use a modifiedpolynomial for calculating the checksum and to transmit it in the CRCfield. This has the advantage that the reliability of error detection ismaintained even for larger transmitted quantities of data. In oneparticularly advantageous exemplary embodiment, multiple calculations ofchecksums are started in parallel at the beginning of a message and, asa function of the satisfaction of a, possibly the same, switchovercondition and/or the content of the data length code, a decision is madeas to which result of one of these calculations is used or transmittedin the CRC field. This makes it possible to transmit the information asto whether a message is transmitted according to the standard-conformingmethod or according to the method modified in accordance with thepresent invention along with the message, without informing therecipient in advance about the used method. The checksums for checkingthe correct data transmission exist for both methods and may beevaluated as needed.

The satisfaction of the switchover conditions is communicated to therecipients by one or multiple identifiers. Here it is particularlyadvantageous if at least one of the identifications occurs by a firstidentification bit, the position of which is between the last bit of theidentifier and the first bit of the data length code and at the positionof which in messages according to the CAN standard ISO 11898-1 there isa bit having a defined value.

It is furthermore advantageous that possibly existing stuff bits, whichappear before the CRC field in the message, are also included in thecalculation of the checksum. This further improves the reliability ofthe data transmission or the detection probability for data transmissionerrors.

If the method is further combined with a switchover of the bit length,for example for at least the bits of the data field and the CRC field,then the further advantage is obtained that a greater quantity of datais transmitted in accelerated fashion than is the case when the datafield is limited to 8 bytes. This further increases the average datatransmission rate of the bus system. In one advantageous development,the messages having a shortened bit length in this case are identifiedby an identifier bit in the control field. This allows the switchover ofthe bit length to occur independently of the switchover of the CRCcalculation or the size of the data field, and it is possible to reactflexibly to the prevailing conditions of the bus system.

The method is advantageously applicable in the normal operation of amotor vehicle for transmitting data between at least two control unitsof the motor vehicle, which are connected via a suitable data bus. Itmay equally be used advantageously during the manufacturing ormaintenance of a motor vehicle for transmitting data between aprogramming unit connected to a suitable data bus for programmingpurposes and at least one control unit of the motor vehicle that isconnected to the data bus. It is also advantageously usable in theindustrial field when larger data quantities must be transmitted forexample for control purposes. Particularly if a reduced data rate mustbe applied during the arbitration due to the length of the transmissionroute so that all subscribers have the opportunity to access the bus,the method makes it possible, in particular in combination with theswitchover of the length of the data field and the reduction of the bitlength, to achieve a higher data transmission rate.

An additional advantage is that a standard CAN controller only needs tobe modified minimally in order to be able to operate in accordance withthe present invention. A communications controller according to thepresent invention, which is also able to work as a standard CANcontroller, is only negligibly larger than a conventional standard CANcontroller. The associated application program does not need to bemodified, and even then advantages in the speed of data transmission arealready achieved.

Advantageously, substantial portions of the CAN conformance test (ISO16845) may be adopted. In one advantageous development, the transmissionmethod according to the present invention may be combined with thesupplements of TTCAN (ISO 11898-4).

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described ingreater detail below with reference to the accompanying drawings.

FIG. 1a shows the two alternatives for the structure of messages in theCAN format according to the CAN standard ISO 11898-1 from the relatedart.

FIG. 1b shows the two analogous alternatives for the format of therespective messages modified in accordance with the present invention.

FIG. 2 represents various possibilities of how, according to the presentinvention, the content of the data length code may be interpreted in amanner deviating from the CAN standard ISO 11898-1.

FIG. 3 schematically shows an exemplary embodiment for the receivingprocess, according to the present invention, in a subscriber station ofthe bus system.

FIG. 4 schematically shows an exemplary embodiment for the receivingprocess, according to the present invention, in a subscriber station ofthe bus system.

FIG. 5 shows two examples for the format of messages modified inaccordance with the present invention, in which additionally a differentbit length is used in defined areas within the message.

FIG. 6 shows another possibility for identifying the messages having ashortened bit length in accordance with the present invention.

FIG. 7 shows a receiving process that is modified compared to FIG. 3, inwhich additionally a switchover is made between the states of Fast CANArbitration and Fast CAN Data as a function of the second identificationBRS.

DETAILED DESCRIPTION

FIG. 1a shows the structure of messages as they are used in a CAN busfor data transmission. The two different formats “standard” and“extended” are shown. The method according to the present invention isequally applicable to both formats.

The message begins with a “start of frame” (SOF) bit, which signals thebeginning of the message. This is followed by a section that is usedprimarily for identifying the message and on the basis of which thesubscribers of the bus system decide whether they receive the message ornot. This section is called an “arbitration field” and contains theidentifier. This is followed by a “control field” containing, amongother things, the data length code. The data length code containsinformation about the size of the data field of the message. This isfollowed by the actual “data field”, which contains the data to beexchanged between the subscribers of the bus system. This is followed bythe “CRC field” having the 15-bit checksum and a delimiter, andsubsequently two “acknowledge” (ACK) bits, which signal to the senderthe successful reception of a message. The message is concluded by an“end of frame” (EOF) sequence.

In the standard CAN transmission method, the data field may contain amaximum of 9 bytes, that is, 64 bits of data. According to the standard,the data length code comprises four bits, which is to say that it canassume 16 distinct values. Of this value range, in today's bus systems,only eight different values are used for the various sizes of the datafield from 1 byte to 8 bytes. A data field of 0 bytes is not recommendedin standard CAN, and sizes above 8 bytes are not permitted. Theassignment of the values of the data length code to the sizes of thedata field is shown in FIG. 2 in the standard CAN column.

In FIG. 1b , in an analogous representation, the modified messages to betransmitted in accordance with the present invention are juxtaposed,respectively derived from the two standard formats.

In the transmission method modified in accordance with the presentinvention, the data field may also contain more than 8 bytes, namely, inthe represented exemplary embodiment, up to K bytes. In contrast tostandard CAN, additional values assumable by the data length code areused to identify larger data fields. For example, the four bits of thedata length code may be used to represent the values from zero to 15bytes. Other assignments may also be made, however, one possibilitybeing the use the value of the data length code DLC=0b0000, which isnormally not used in today's CAN messages, for another possible size ofthe data field, for the size of 16 bytes for example. These twopossibilities are shown in FIG. 2 in tabular form as DLC 1 and DLC 2.The maximum size of data field K in this case has the value 15 and 16,respectively.

Another possibility is that for the values of the data length codegreater than 0b1000 and up to 0b1111, the associated sizes of the datafield grow respectively by 2 bytes for example. This case is shown inthe table as DLC 3. The maximum size of data field K in this variantreaches the value 24. By selecting a greater increment, for example 4bytes, greater data fields would be achievable accordingly.

In the DLC 3 example, one additional modification has been made: thevalue DLC=0b0000 is used in this exemplary embodiment by remote frames.Standard CAN, by contrast, provides for transmitting a remote frame atthe same value of the DLC as the message transmitted as a reaction tothe remote frame. The modification described here ensures that remoteframes having a different DLC and an identical identifier cannot betransmitted, which (cf. ISO 11898-1, chap. 10.8.8) may result ininextricable collisions.

In the exemplary embodiments of the method shown in tabular form in FIG.2 in columns DLC 1, DLC 2 and DLC 3, the assignment of the values from0b0001 through 0b1000 of the data length code to the sizes of the datafield between 1 byte and 8 bytes corresponds to the assignment instandard CAN. In a simple manner, this makes it possible to achievecompatibility to standard CAN, that is, to design the communicationscontroller such that it works in a standard-conforming manner in astandard CAN bus system, while allowing greater data fields in themessages in a bus system modified in accordance with the presentinvention. It is also possible, however, to perform a completely newassignment of the possible values of the data length code to admissiblesizes of the data field. An example of this is provided as DLC 4 in FIG.2. In this case, the achieved maximum size K of the data field is 30bytes.

To ensure that such a communications controller is able to determine inwhat manner it must interpret the contents of the data length code, itis advantageous for the communications controller to recognizeindependently whether the communication of the bus system occursaccording to standard CAN or according to the method of the presentinvention. One possibility for this is to use a reserved bit within thearbitration field or the control field for identification such that fromthis first identification K1 the communications controller is able toderive a first switchover condition UB1, on the basis of which itselects the transmission method. For example, the second bit of thecontrol field indicated by r0 in FIG. 1b may be used for identification.

It is also possible, however, to select the determination as a functionof the identifier format. For standard addressing, one option foridentifying the messages according to the present invention is thus toinsert a recessive EDL (extended data length) bit into the control fieldin the position of the r0 bit that is always dominant in standard CAN.For extended addressing, the recessive EDL bit in the control field cantake the position of the r1 bit that is always dominant in standard CAN.

Another possibility is to use the SRR bit, which in standard CAN mustalways be transmitted recessively, but which is accepted also dominantlyby the bus subscribers receiving the message. It is also possible toevaluate bit combinations to determine the first switchover conditionUB1.

Another possibility would be to prescribe the use of the extended formatfor the transmission method modified in accordance with the presentinvention. Messages in the extended format are recognized by bussubscribers on the basis of the value of the IDE bit (cf. FIG. 1a ), andthis bit could at the same time represent the first switchover conditionUB1 such that the modified transmission method is always used forextended messages. Alternatively, it would also be possible in extendedmessages to use the reserved bit r1 as a first identification K1 or forderiving the first switchover condition UB1. As explained further below,the reserved bit may also be used for deriving a second switchovercondition UB2 for switching between more than two different sizes of thedata field or assignments between values of the data length code anddata field sizes. Such a second switchover condition or even multipleadditional switchover conditions may flexibly

Alternatively, it is also possible to apply the method in suitablecommunications controllers that are not also designed forstandard-conforming CAN communication. In this case, the determinationof the mentioned first switchover condition UB1, for example as afunction of a suitable identification K1 of the messages, may also bedropped. In this case, the communications controllers rather operateexclusively according to one of the described methods and areaccordingly usable only in bus systems in which such communicationscontrollers according to the present invention are used exclusively.

If, as provided in the present invention, the data field of messages isenlarged, then it may be practical to adapt also the method utilized forthe cyclic redundancy check (CRC) in order to obtain a sufficientimmunity against error. In particular, it may be advantageous to use adifferent CRC polynomial, for example of a higher order, and accordinglyto provide a CRC field of a deviating size in the messages modified inaccordance with the present invention. This is indicated in FIG. 1b bythe fact that the CRC field of the messages of the present invention hasa length of L bits in the example provided, it being possible for L tobe unequal to, in particular greater than, 15, in deviation fromstandard CAN.

The use of a modified method for calculating the CRC checksum may besignaled to the bus subscribers by a third identification K3, whichrepresents a third switchover condition UB3. This identification K3 andthe third switchover condition UB3, however, may also agree with thefirst identification K1 and/or switchover condition UB1. Here too, aswas described further above, the reserved bit r0 from FIG. 1b may beused for identification for example, or the SRR bit may be used. A useof the IDE bit in combination with the application of the method inextended messages or even of the r1 bit is also possible.

In standard CAN controllers, the CRC code of CAN messages to betransmitted is generated by a feedback shift register, the seriallytransmitted bits of the message being fed sequentially into its input.The width of the shift register corresponds to the order of the CRCpolynomial. The CRC encoding occurs by combining the register contentwith the CRC polynomial during the shift operations. When CAN messagesare received, the serially received bits of the message are accordinglyshifted into the CRC shift register. The CRC test is successful if atthe end of the CRC field all bits of the shift register are at zero. TheCRC code generation in the sending case and the CRC test in thereceiving case both occur in hardware without requiring an interventionof the software. A modification of the CRC encoding thus does not affectthe application software.

In the standard CAN protocol, the stuff bits within the CAN messages(cf. ISO 11898-1, chap. 10.5) are not included in the calculation orchecking of the CRC code (cf. ISO 11898-1, chap. 10.4.2.6: “ . . . thebit stream given by the destuffed bit sequence . . . ”). This has theconsequence that in rare cases two bit errors in one message are notdetected even though the CRC should as such detect up to five randomlydistributed bit errors in one message. This may occur when, as a resultof the bit errors, stuff bits transform into data bit and vice versa(cf. Unruh, Mathony and Kaiser: “Error Detection Analysis of AutomotiveCommunication Protocols”, SAE International Congress, No. 900699,Detroit, USA, 1990).

In the transmission method modified in accordance with the presentinvention, by contrast, the CRC encoding may be changed such that thestuff bits within the message are also included in the calculation orchecking of the CRC codes. That is to say that in this exemplaryembodiment the stuff bits belonging to the arbitration field, controlfield and data field are treated as part of the data to be protected bythe cyclic redundancy check. As in standard CAN, the stuff bits of theCRC field are disregarded.

In one possible exemplary embodiment, the communications controller isdesigned such that it is compatible with the standard CAN, that is, itworks in a standard-conforming fashion in a standard CAN bus system,while in a bus system modified in accordance with the present inventionit allows for larger data fields in the messages on the one hand and onthe other hand also performs the adapted calculation and checking of theCRC code.

Since at the start of the reception of a message it is not yet clearwhether a standard-conforming CAN message or a message modified inaccordance with the present invention is received, two CRC shiftregisters are implemented in a communications controller according tothe present invention, which work in parallel. Following the receptionof the CRC delimiter, when the CRC code is evaluated in the receiver, itis clear from the third identification K3 according to the presentinvention or from the third switchover condition UB3 derived from theidentification or the content of the data length code, for example,which transmission method was used, and the shift register associatedwith this transmission method is then evaluated. As already explainedabove, the third switchover condition UB3 may agree with the firstswitchover condition UB1, which concerns the size of the data field andthe interpretation of the data length code.

To be sure, it is already clear for the sender at the beginning ofsending a message according to which transmission method a transmissionis to occur. Since it could happen, however, that the arbitrationregarding bus access is lost and the started message is not sent, butinstead a different message is received, both CRC shift registers areactivated in parallel in this case as well.

The described implementation of two CRC shift registers working inparallel also allows for another improvement:

The CRC polynomial of the standard CAN protocol(x15+x14+x10+x8+x7+x4+x3+1) is designed for a message length of lessthan 127 bits. If messages transmitted in accordance with the presentinvention also use longer data fields, then it is practical to use adifferent, in particular longer, CRC polynomial in order to maintaintransmission reliability. The messages transmitted in accordance withthe present invention accordingly receive a modified, in particularlonger, CRC field. In ongoing operation, the communications controllersswitch dynamically between the two CRC shift registers, that is, betweenthe standard CAN-conforming shift register and the shift register of thepresent invention, in order to use the respectively fitting polynomial.

Of course, more than two shift registers and accordingly more than twoCRC polynomials may also be used, graduated as a function of the lengthof the data field or the desired transmission reliability. In this case,if a compatibility with the standard CAN is to be maintained, thecorresponding identification and the associated switchover conditionmust be adapted. For example, a first switchover condition UB1 could betriggered by the reserved r0 bit or the SRR bit in FIG. 1b , whichindicates a switchover to longer data fields, for example according toDLC 1 in FIG. 2, and an associated second CRC polynomial. Additionally,for example for messages in the extended format, a second switchovercondition UB2 could be triggered by the reserved bit r1 or the IDE bitin FIG. 1b , which indicates the switchover to another set of data fieldsizes, for example DLC 3 from FIG. 2, and a third CRC polynomial. It isalso possible, however, to use additional corresponding switchoverconditions and identifications, which are inserted or taken up into themessage after first identification K1, to increase further the variationpossibilities with respect to the available valid data field sizes.Following the reception of the first identification or evaluation of theassociated switchover condition, the subscribed data processing unitsmay then adapt their transmission process accordingly and evaluate thisadditional second identification or further identifications orswitchover conditions and adapt the transmission method to the datafield size respectively ascertained from the switchover conditions andthe content of the data length code.

It is incidentally also possible that first switchover condition UB1switches over to the option of longer data fields and the correspondinginterpretation of the content of the data length code, perhaps via thereserved bit r0 or the SRR bit, and that the ascertainment of the thirdswitchover condition UB3 and accordingly the selection of the CRCpolynomial to be evaluated for the CRC check then occurs as a functionof the content of the data length code. Third switchover condition UB3may accordingly also assume more than two values. For example, the datafield sizes could be selected according to DLC 3, that is, assume thevalues 0 (for remote frames) 1, . . . , 8, 10, 12, 14, 16, 18, 20 and 24bytes, and three CRC polynomials could then be calculated in parallelvia suitable shift registers, for example the standard CRC polynomialfor data fields up to 8 bytes, a second CRC polynomial for data fieldsup to 16 bytes and a third CRC polynomial for data fields up to 24bytes.

FIG. 3 shows in a simplified representation a segment of the receivingprocess according to the present invention, as it occurs in a subscriberstation of the bus system. The case shown is one in which acompatibility with the standard CAN is achieved in that the behavior ofthe communications controller is adapted as a function of the firstswitchover condition UB1. Although FIG. 3 shows a representation typicalfor describing program runs in software, the method is equallycompletely suited for implementation in hardware.

The subscriber station is first in a bus-scanning state as long as thereis no communications traffic on the bus. Query 302 is thus waiting for adominant bit on the bus. This bit marks the start of a new message.

As soon as the start of a new message has been determined, thecalculation of the at least two checksums to be calculated in parallelbegins in Block 304. The first checksum corresponds to the CRCcalculation of the standard CAN, while the second checksum is calculatedaccording to the new method. In the calculation of the second checksum,the stuff bits are included in the exemplary embodiment shown, whereasthis is not the case in the calculation according to the standard CAN.It is also possible, however, not to take the stuff bits into accounteven for calculating the second checksum, in analogy to the standardCAN.

Subsequently, beginning with step 306, the additional bits of themessage following the SOF bit are received, beginning with thearbitration field. If multiple bus subscribers want to send out amessage, then the bus subscribers negotiate among themselves inaccordance with the usual method of the standard CAN which bussubscriber gains the access to the bus. Block 306 indicates thereception of all bits until first identification K1 has been received orfirst switchover condition UB1 has been determined. In the exampleslisted, first switchover condition UB1 is ascertained from thearbitration field, for example from the SRR bit or the IDE bit, or fromthe control field, for example from a reserved bit of the same (cf. FIG.1). Subsequently, additional bits of the message may be received inblock 308 until, beginning with a specific bit of the message as afunction of the ascertained first switchover condition UB1, a differentmethod is followed. This split into different methods is ensured by acorresponding query or branching 310, as shown in the following by wayof example.

If it is known at branching 310, for example following the reception ofthe first two bits of the control field, that according to firstswitchover condition UB1 the communication occurs in accordance with theCAN standard (the path of FIG. 3 indicated by “1”), then the additionalbits of the control field are read in step 312. From these bits, thedata length code is evaluated according to the CAN standard andsubsequently, in step 316, the associated set of data, 8 bytes maximum,is received in accordance with the data field. The 15-bit CRC field isthen received in step 320. If it is known at branching 324 that the CRCchecksum transmitted by the sender agrees with the CRC checksumascertained by the receiver, then a dominant acknowledge bit istransmitted in block 328. It should be noted that in this case thestandard-conforming CRC checksum is compared since the communicationoccurs according to the CAN standard. If no agreement is ascertained,the acknowledge bit is transmitted recessively (Block 330). This isfollowed by the ACK delimiter and EOF bits (see FIG. 1b , not shown inFIG. 3).

If, by contrast, it is known at branching 310, for example following thereception of the first two bits of the control field, that according tofirst switchover condition UB1 the communication method modified inaccordance with the present invention is to be applied (the path of FIG.3 indicated by “2”), then the additional bits of the control field areread in step 314. From the result, the data length code is ascertainedaccording to the new interpretation, for which some examples are listedin tabular form in FIG. 2. In block 318, the corresponding quantity ofdata is received, that is, up to 15 bytes for the example DLC 1 from thetable in FIG. 2, up to 16 bytes for the example DLC 2, up to 24 bytesfor the example DLC 3, and up to 30 bytes for the example DLC 4. The CRCfield deviating in accordance with the present invention, being inparticular longer, is received in block 322. If it is known at branching324 that the CRC checksum transmitted by the sender agrees with the CRCchecksum ascertained by the receiver, then a dominant acknowledge bit istransmitted in block 328, the comparison in this case being based on theCRC checksum that deviates in accordance with the present invention.Otherwise, the acknowledge bit is transmitted recessively (Block 330).The ACK delimiter and the EOF bits follow in step 332 or 334. This endsa receiving process for a message.

FIG. 3 shows the case in which third switchover condition UB3, whichdetermines the CRC to be used, agrees with first switchover conditionUB1, which concerns the size of the data field and the interpretation ofthe data length code. Thus, prior to the reception 320 or 322 of the CRCchecksums, no additional query was made as to which CRC is to bereceived according to the third switchover condition UB3 and to beevaluated for branching 324. As shown in FIG. 4, this additional querymay be integrated into the process by a simple modification of the flowchart from FIG. 3.

In the reception process modified in this manner, as shown in FIG. 4,following the reception of the number of data bytes of the data fieldexpected according to the information from the data length code, queryor branching 410 ascertains the value of switchover condition UB3 inblock 316 or 318. As described earlier, this information may have beenascertained for example from the corresponding third identification orfrom the content of the data length code. In the example shown, thereare three different values for third switchover condition UB3: A, B andC. Depending on the value of switchover condition UB3, a differentnumber of bits of the CRC field is then read in blocks 420, 422 and 424,for example 15 bits for the value A, 17 bits for the value B and 19 bitsfor the value C. Subsequently, a check is performed in branching 324analogous to FIG. 3 whether the CRC checksum transmitted by the senderagrees with the CRC checksum ascertained by the receiver, and thefurther method is selected accordingly.

FIG. 5 shows once more the structure of messages in the two possiblevariants, the standard format and the extended format, for additionalexemplary embodiment of the transmission method according to the presentinvention. Areas are drawn for both variants in FIG. 5, in which aswitchover is made between two states, here indicated as Fast CANArbitration and Fast CAN Data. This switchover between the two states inthis example has the effect that following the conclusion of thearbitration the bit lengths are shortened for a portion of the message,in particular for the data field and the CRC field, and thus theindividual bits are transmitted via the bus more quickly. This makes itpossible to shorten the transmission time for a message compared to thestandard-conforming method. The associated change in the bit length intime may implemented by using at least two different scaling factors forsetting the bus time unit relative to a smallest time unit or theoscillator clock pulse in ongoing operation. The switchover of the bitlength as well as the corresponding change of the scaling factor areshown in FIG. 5 also by way of example.

The transition between the states of Fast CAN Arbitration and Fast CANData may occur as a function of a fourth switchover condition UB4, whichcorresponds to a fourth identification K4 of the messages, which signalsto the subscribers of the data transmission that the shortened bitlength is used. In the exemplary embodiment shown here, the chosenposition of this identification K4 is the “reserved bit” r0, which istransmitted before the data length code. It thus corresponds to apossible position of first identification K1, which corresponds to firstswitchover condition UB1 and indicates the possible use of longer datafields and a modified interpretation of the data length code, and alsoof third identification K3, which corresponds to a modified CRCcalculation.

Another possibility for identifying the messages having a shortened bitlength in accordance with the present invention is shown in FIG. 6.Here, the messages having potentially longer data fields (associated:first identification K1) and modified CRC calculation (associated: thirdidentification K3) are indicated by a recessive EDL (extended datalength) bit, which takes the place of a bit transmitted dominantly instandard CAN messages and replaces this bit or shifts it backwards byone position. For the standard addressing, the EDL bit assumes thesecond position in the control field and shifts the always dominant r0bit located there by one position. For the extended addressing, the EDLbit in the example shown assumes the first position of the control fieldand replaces the reserved r1 bit located there, which is alwaystransmitted dominantly in standard CAN.

If multiple different assignments between the content of the data lengthcode and the size of the data field are to be usable, additionalidentifiers may be incorporated into the messages of the presentinvention in bit positions within the control field, which are thenevaluated for selecting the respective assignment of values of the datalength code to sizes of the data field.

The fourth identification K4, which announces the use of the shortenedbit length, is represented by the insertion of an additional, recessiveBRS (bit rate switch) bit into the control field of messages accordingto the present invention, which are indicated by the EDL bit. In theexemplary embodiment shown here, the position of the BRS bit is thefourth (standard addressing) or third (extended addressing) position inthe control field.

The messages bear the label “CAN FD Fast”. For the two possibleaddressing variants of messages, the standard format and the extendedformat, areas are drawn in FIG. 6, in which a switchover is made betweentwo states indicated as Fast CAN Arbitration and Fast CAN Data. Thisswitchover between the two states has the effect, as already described,that the bit lengths are shortened for the corresponding part of themessage and thus the individual bits are transmitted more quickly acrossthe bus. This makes it possible to shorten the transmission time for amessage compared to the standard-conforming method. The transitionbetween the states of Fast CAN Arbitration and Fast CAN Data occurs inmessages that have the first or third identification EDL, as a functionof the fourth identification BRS, which signals to the subscribers ofthe data transmission that the shortened bit length is being used.

In the case shown, in which the first identification EDL is thusfollowed by the second identification BRS, messages are transmitted inthe transmission method according to the present invention, the bitlength of which is markedly shortened, the data field size of which isexpandable to values above 8 bytes, and the CRC of which is adapted tothe larger data field. A substantial increase of the transmissioncapacity via the bus system is thus achieved while the transmissionreliability is at the same time improved.

In the example shown, the faster transmission begins immediately afterthe transmission of the associated identification and is endedimmediately after reaching the bit defined for the reverse switchover orwhen a reason for starting an error frame was detected.

FIG. 7 shows a receiving process that is modified compared to FIG. 3, inwhich additionally a switchover is made between the states of Fast CANArbitration and Fast CAN Data as a function of the second identificationBRS.

If it is known at branching 310, for example after receiving the secondbit of the control field as the recessive EDL bit, that thecommunication method modified in accordance with the present inventionis to be used, then the next bits of the control field are read in block408. If the bit used for the second identification, for example thefourth bit BRS of the control field extended in accordance with thepresent invention, is received with the provided value, for examplerecessively, then the state Fast CAN Data is assumed for example at thesample point of this bit, that is, a switchover is made to the shortenedbit length (path “C”). If the respective bit has the opposite value,that is, dominant in this example, then the bit length is not shortened(path “B”). In blocks 412 or 414, the remaining bits of the controlfield including the data length code are received as well as the datafield according to the size information from the data length code.Reception is at normal bit length in block 412, and at shortened bitlength in block 414. In blocks 416 or 418, the CRC field deviating inaccordance with the present invention, being in particular longer, isread in. In block 418, at the final bit of the CRC field, the CRCdelimiter, a switchover is made back to the state of Fast CANArbitration at a normal bit rate. Subsequently, a check is performed atbranching 324 analogous to FIG. 3 to determine whether the CRC checksumtransmitted by the sender agrees with the CRC checksum ascertained bythe receiver, and the further method is selected accordingly, as wasalready the case in FIG. 3.

The following calculation illustrates the utility in terms of theachieved data transmission rate of the exemplary embodiment shown inFIG. 5 in combination with the exemplary embodiment of the methodindicated by DLC 3 having a modified size of the data field: We assume alength of the data field of 24 bytes, data frames in the standard formatfeaturing 11-bit addressing, as well as a baud rate of 500 kbit/s.Moreover, it is assumed that the scaling factor according to “reservedbit” r0 is increased by a factor of four. In this case, the bit lengthwould thus be reduced from 2 microseconds to 0.5 microseconds accordingto “reserved bit” r0. Ignoring possible stuff bits, in this example, perdata frame, 27 bits (SOF, identifier, RTR, IDE, r0, ACK field, EOF,intermission) are transmitted at the normal bit length and 212 bits(DLC, data, CRC, CRC delimiter) are transmitted at the shortened bitlength, a 15-bit CRC having still been assumed here, which, however, inaccordance with the present invention could be replaced by a longer CRC.

Under the given boundary conditions, an effective transmissionperformance of 293 bits in 160 microseconds is achieved, which at anidentical assumed bus capacity utilization corresponds to a datatransmission rate that is increased by a factor of 3.7 compared tounmodified standard CAN transmission. Additionally, the ratio of usefuldata (data field) to protocol overhead shifts advantageously.

The method is suitable in the normal operation of a motor vehicle fortransmitting data between at least two control units of the motorvehicle, which are connected via a suitable data bus. It may equally beused advantageously during the manufacturing or maintenance of a motorvehicle for transmitting data between a programming unit connected to asuitable data bus for programming purposes and at least one control unitof the motor vehicle that is connected to the data bus.

It is furthermore also possible to use the method in industrialautomation, that is, for example in the transmission of controlinformation between distributed control units interconnected by the bus,which control the course of an industrial manufacturing process. In thisenvironment there may also be very long bus lines and it may beparticularly practical to operate the bus system for the arbitrationphase at a relatively long bit length, for example at 16, 32 or 64microseconds, so that the bus signals, as required, are able topropagate through the entire bus system during the arbitration process.Subsequently, a switchover to shorter bit lengths may be performed forpart of the message, as described, so as not to let the averagetransmission rate drop too low.

Overall, the method represents a transmission method that ischaracterized by the fact that a standard CAN controller only needs tobe modified minimally in order to work in accordance with the presentinvention. A communications controller according to the presentinvention, which is also able to work as a standard CAN controller, isonly negligibly larger than a conventional standard CAN controller. Theassociated application program does not need to be modified, and eventhen advantages in the speed of data transmission are already achieved.Using the extended size of the data field and the associated DLC and CRCmakes it possible to increase the speed of data transmission further,while adaptations in the application software are minimal.Advantageously, substantial portions of the CAN conformance test (ISO16845) may be adopted. It is also possible to combine the transmissionmethod according to the present invention with the supplements of TTCAN(ISO 11898-4).

Where reference was made to ISO standards in the previous description ofthe present invention, the version of the respective ISO standard validon the filing date is to be treated as the related art.

What is claimed is:
 1. A method for serial data transmission in a bussystem having at least two subscribed data processing units thatexchange messages via a bus in the bus system, each of the messageshaving a logical structure including a start-of-frame bit, anarbitration field, a control field, a data field, a Cyclical RedundancyChecking (“CRC”) field, an acknowledge field and an end-of-framesequence, the control field including a data length code, which containsinformation about the length of the data field, the method comprising:interpreting, in a transmitted message, values of bits of the datalength code: i) in deviation from the Controller Area Network (“CAN”)standard International Organization for Standardization (“ISO”) 11898-1for determining a number of bits of the data field when a firstswitchover condition is satisfied, and ii) in conformance with the CANstandard ISO 11898-1 for determining the number of bits of the datafield when the first switchover condition is not satisfied, whereinsatisfaction of the first switchover condition is determined based on atleast one switchover bit included in a portion of the logical structureof the transmitted message, the portion not including the data lengthcode.
 2. The method as recited in claim 1, wherein, as a function of avalue of the first switchover condition, each of possible valuecombinations of the bits of the data length code is assigned to one ofadmissible sizes of the data field.
 3. The method as recited in claim 1,wherein the messages, in which the data field of the messages maycomprise more than eight bytes in deviation from the CAN standard ISO11898-1 and the values of the bits of the data length code areinterpreted at least partially in deviation from the CAN standard ISO11898-1 for determining the size of the data field, are distinguishableby a first identification of the at least one switchover bit in at leastone of the arbitration field or the control field of CANstandard-conforming messages.
 4. The method as recited in claim 3,wherein the first identification is evaluated in the subscribed dataprocessing units to ascertain the first switchover condition and thereceiving process is adapted to the size of the data field as a functionof the first switchover condition.
 5. The method as recited in claim 3,wherein the first identification occurs by a first identification bit, aposition of which is between a last bit of the identifier and a firstbit of the data length code and at the position of which in messagesaccording to the CAN standard ISO 11898-1 there is a bit having adefined value.
 6. The method as recited in claim 5, wherein in thecontrol field a second identification bit follows upon the firstidentification bit for a second identification or additionalidentification bits follow upon the first identification bit foradditional identifications.
 7. The method as recited in claim 1,wherein, if the first switchover condition is satisfied, the data fieldmay comprise 16 different sizes and 16 value combinations of the bits ofthe data length code are assigned to the 16 different sizes of the datafield.
 8. The method as recited in claim 1, wherein, when the firstswitchover condition is satisfied, a maximum possible size of the datafield is greater than 16 bytes.
 9. The method as recited in claim 1,wherein values between 0b0001 and 0b1000 of the data length code areused for the sizes of the data field between 1 and 8 bytes in accordancewith the CAN standard ISO 11898-1 and, if the first switchover conditionis satisfied, remaining values of the data length code are used foradditional admissible sizes of the data field up to a maximum possiblesize.
 10. The method as recited in claim 1, wherein, when a secondswitchover condition is satisfied or multiple additional switchoverconditions are satisfied, the bits of the data length code areinterpreted according to at least one additional switchoverinterpretation from the CAN standard ISO 11898-1.
 11. The method asrecited in claim 10, wherein the messages, in which, when a secondswitchover condition is satisfied or multiple additional switchoverconditions are satisfied, the bits of the data length code are to beinterpreted at least partially in deviation from the CAN standard ISO11898-1 and in deviation from the assignment when the second switchovercondition is not satisfied or the additional switchover conditions arenot satisfied, are recognizable by a second identification or additionalidentifications in at least one of the arbitration field or the controlfield.
 12. The method as recited in claim 11, wherein the secondidentification is evaluated or the additional identifications areevaluated in the subscribed data processing units to ascertain thesecond switchover condition or the additional switchover conditions andthe receiving process is adapted to the size of the data field as afunction of the result of the evaluation.
 13. The method as recited inclaim 11, wherein the second identification appears only in messageswhose arbitration field has the format of a CAN message in extendedformat, and/or agrees with the identification of the extended format.14. The method as recited in claim 1, wherein, as a function of a valueof a third switchover condition, the CRC field of the messages may haveat least two different numbers of bits, at least one of the validnumbers of bits in the CRC field being a number of bits deviating fromthe CAN standard ISO 11898-1, a generator polynomial deviating from theCAN standard ISO 11898-1 being used for determining a content of the CRCfield, which has a deviating number of bits.
 15. The method as recitedin claim 14, wherein the messages, in which, as a function of the valueof the third switchover condition, the CRC field of the messages mayhave at least two different numbers of bits, are recognizable by a thirdidentification in at least one of the arbitration field or the controlfield, it being possible for the third identification to agree with thefirst identification.
 16. The method as recited in claim 15, wherein thevalue of the third switchover condition is ascertained in the subscribeddata processing units as a function of the third identification oragrees with the first switchover condition or is derived from at leastone of the first switchover condition or the content of the data lengthcode, the receiving process being adapted to the size of the CRC fieldas a function of the value of the third switchover condition.
 17. Themethod as recited in claim 14, wherein at the beginning of a message thecalculation of at least two CRC checksums is started in parallel usingdifferent generator polynomials and a decision is made as a function ofthe value of the third switchover condition as to which result is usedfrom one of the CRC calculations that were started in parallel.
 18. Themethod as recited in claim 14, wherein, when the third switchovercondition is satisfied, possible stuff bits within the segments of themessage in front of the CRC field are also taken into account in atleast one performed CRC calculation.
 19. The method as recited in claim1, wherein as a function of a value of a fourth switchover condition, abit length in time may assume at least two different values within amessage, the bit length in time being greater than or equal to aspecified minimum value of approximately one microsecond for at leastone first specifiable area within the message and the bit length in timein at least one second specifiable area within the message having areduced value in comparison to the first area.
 20. The method as recitedin claim 19, wherein the messages, in which the bit length in time mayassume at least two different values within a message as a function ofthe value of the fourth switchover condition, are recognizable by afourth identification in at least one of the arbitration field or thecontrol field, it being possible for the fourth identification to agreewith at least one of the first or third identification.
 21. The methodas recited in claim 20, wherein the fourth identification occurs by anadditional identification bit, which is located between a firstidentification bit and a first bit of the data length code.
 22. Themethod as recited in claim 19, wherein the at least two different valuesof the bit length in time within a message are implemented by using atleast two different scaling factors for setting a bus time unit relativeto a smallest time unit or an oscillator clock pulse in ongoingoperation.
 23. The method as recited in claim 19, wherein the value ofthe fourth switchover condition is ascertained in the subscribed dataprocessing units as a function of the fourth identification or agreeswith at least one of the first switchover condition or a thirdswitchover condition or is derived from at least one of the firstswitchover condition or the third switchover condition, the receivingprocess being adapted to the different values of the bit length within amessage as a function of the value of the fourth switchover condition.24. The method as recited in claim 1, wherein the messages aretransmitted in a time-controlled manner in accordance with a methoddescribed in Time-Triggered Controller Area Network (“TTCAN”) standardISO 11898-4.
 25. The method as recited in claim 1 wherein the values ofthe bits of the data length code are interpreted in normal operation ofa motor vehicle or an industrial installation, and wherein the at leasttwo subscribed data processing units include at least two control unitsof the motor vehicle or the industrial installation, which are connectedvia a suitable data bus.
 26. The method as recited in claim 1, whereinthe values of the bits of the data length code are interpreted duringmanufacturing or maintenance of a motor vehicle or an industrialinstallation, and wherein the at least two subscribed data processingunites include a programming unit connected to a suitable data bus forprogramming purposes and at least one control unit of the motor vehicleor the industrial installation that is connected to the data bus. 27.The method as recited in claim 1, wherein the switchover interpretationdeviates from the CAN standard ISO 11898-1 for determining the number ofbits of the data field when the first switchover condition is satisfied.28. A device for serial data transmission in a bus system, comprising:at least two subscribed data processing units that exchange messages viaa bus in the bus system, each of the messages having a logical structureincluding a start-of-frame bit, an arbitration field, a control field, adata field, a Cyclical Redundancy Checking (“CRC”) field, an acknowledgefield and an end-of-frame sequence, the control field including a datalength code, which contains information about a length of the datafield, wherein, when a first switchover condition is: i) satisfied,values of bits of the data length code are interpreted by one of the atleast two subscribed data processing units in deviation from theController Area Network (“CAN”) standard International Organization forStandardization (“ISO”) 11898-1 for determining a number of bits of thedata field, and ii) not satisfied, values of the data length code areinterpreted in conformance with the CAN standard ISO 11898-1 fordetermining the number of bits of the data field, wherein satisfactionof the first switchover condition is determined based on at least oneswitchover bit included in a portion of the logical structure of thetransmitted message, the portion not including the data length code. 29.The device as recited in claim 28, wherein the device is configured toexchange messages, including the transmitted message, by interpretingthe values of bits of the data length code one of in deviation from orin conformance with the CAN standard ISO 11898-1.
 30. The device asrecited in claim 28, further comprising a number of shift registers thatcalculate at the beginning of the transmitted message, in parallel, atleast two CRC checksums using different generator polynomials.
 31. Themethod of claim 30, wherein the at least two different generatorpolynomials have varying lengths relative to one another.
 32. A methodfor serially transmitting data over a bus of a bus system between twosubscribed data processing units communicatively coupled by the bus, themethod comprising: a first one of the two subscribed data processingunits: determining a length of data to be included in a message in adata field, establishing, based on a first switchover condition: (a) acorrelation between a data length code and the determined length of datato be included in the data field that is one of: i) in deviation fromthe Controller Area Network (“CAN”) standard International Organizationfor Standardization (“ISO”) 11898-1 when the first switchover conditionis satisfied, and ii) in conformance with the CAN standard ISO 11898-1when the first switchover condition is not satisfied, and (b) apolynomial that is of a length based on the determined length of data,the polynomial for calculating a checksum, and transmitting a messageincluding: (a) the data length code having the established correlationto the determined length of data included in the data field, and (b) aCyclical Redundancy Checking (“CRC”) field including the polynomial forcalculating the checksum, over the bus to a second one of the twosubscribed data processing units; wherein the message has a logicalstructure, the logical structure including: a start-of-frame bit, anarbitration field, a control field, the data field, the CRC field, anacknowledge field and an end-of-frame sequence, the control fieldincluding the data length code that contains information about thelength of the data field, and wherein satisfaction of the firstswitchover condition is determined based on at least one switchover bitincluded in a portion of the logical structure of the transmittedmessage, the portion not including the data length code.